The antimicrobial action of ultraviolet (UV) radiation is well known. A drawback of existing systems resides in the power consumption and limited lifespan of UV lamps. In order to address this, it is desirable to provide a means to control the intensity of the UV lamp, in order that the lamp intensity may be attenuated adapted to the status of the system.
Low-pressure UV lamps used in disinfection plants comprise a pair of heating filaments or cathodes at either end. A supplied voltage is utilized to heat the cathodes up to a temperature at which an emission of electrons occurs. These electrons can then be used to initiate a glow discharge across the tube causing the gas to radiate by applying a high voltage across the two cathodes. Commonly, an electronic dimmer circuit linked to the UV lamp is used to control its intensity.
It is known that UV lamp cathodes should be pre-heated in order to start the lamp, as explained above. Pre-heating increases the so-called thermionic emission of electrodes, which is enhanced by a suitable surface coating of the cathodes. At too low temperatures, the emission of electrodes necessitates higher voltages, which in turn results in a damage of the coating and hence in a damage of the UV lamp itself.
Pre-heating protects the cathodes and prolongs the lifespan of the UV lamp. In addition it has been shown, that during operation the temperature of the cathodes should remain elevated. Otherwise cathode material is damaged if the temperature of the cathodes is too low. Nowadays, dimming ranges up to 90% are reached, resulting in a lamp output of only 10% of the nominal power output. The parameters of the electric energy to drive lamps under dimmed conditions are usually optimized in a way that the efficiency of the UV light production in terms of radiation output versus power input is optimized. Parameters are voltage, current and pulse length or duty cycle in case of pulse-width modulation. The current under dimmed conditions is so low that it does not generate enough heat when passing the cathodes. Thus to minimise damage to the cathodes, additional heat sources are used, that prevent a cool-down of the cathodes. The drawback of this is, that additional heat sources are complex and costly.